It depends on how things are graded. On a multiple-choice test with four choices per question, someone with no knowledge who guesses randomly will get ~25%. On a true-false test, someone with no knowledge gets ~50%. On a project graded by a human, or a worksheet whose answers are real numbers, someone with no knowledge and a hard-eyed grader might well get 0%. Different classes will have different proportions of these things that contribute to the overall grade (at least, I haven't heard of any requirement that classes have the same proportions of such). The simple approach of summing total points achieved over each graded item, divided by total points possible, is straightforward to calculate, but I think there's no mathematical justification for choosing one percentage-based grading scale and applying it uniformly to all classes.